In an elevator control system, it is essential to detect the position of the elevator car. There are several methods for doing this.
Simple elevators use hoistway vanes or cams to indicate floor location and count these as the car moves up and down the hoistway to determine position. These schemes require an initialization at an absolute position, usually by sensing a contact at a terminal landing, when the system is powered up. The motion control system then requires the car to be initialized every time the power is removed, then reapplied to the car controller. One way in which this is presently done is by moving the car to a terminal landing and sensing a limit switch. The position information may be stored in non-volatile memory so that it is not lost when power is removed. A problem exists, however, such that if the car is in motion and approaching a vane or cam when power is lost, a position error of one floor may exist when power is reapplied. This is due to the car passing over the vane after power is removed before coming to rest. Because of recent changes to the B44, Canadian Elevator Code, a correction to a terminal landing is not allowed when power is lost and reapplied.